There are many photographic situations where the
range of luminosity values between shadows and highlights is too great to
be accommodated in a single exposure. This is just as true with modern
digital imaging as it always has been with film.

With film the solution is to use graduated filters
to lower the luminosity range, typically using the upper dark area of the
filter to reduce the luminosity of the sky while leaving the foreground
unaffected, resulting in an image which holds detail throughout. It many
landscape images this approach yields good results, provided of course you
have the correct filter strength to hand. In some cases however it is
difficult to use a grad filter without its use being obvious, this is
particularly true where the transition between dark and light is not a
reasonably straight line. Shots using grad filters which have an uneven
horizon, such as mountain ranges, or which have trees or other elements
poking up into the sky (like the image below) can be impossible to achieve
with a grad filter without the telltale darkening in the upper part of the
image.

Digital imaging pretty much does away with the need
to use graduated filters to hold detail in skies, and allows any scene
with a very high contrast range to be visualised by combining separate
exposures which individually carry information on different parts of the
total image. There are several ways of achieving the blending of two (or
more) images to encompass a large contrast range and there are a number of
web pages devoted to discussion of image blending. The summary on Luminous
Landscape is a good place to start if you are new to this topic.

Blending
procedure

shadows exposure (1/8 @ f16)

highlights exposure (1/30 @ f16)

The two images above of Llandaff Cathedral were taken
at 8 o'clock in the morning at the end of September. I had previously
taken one exposure at the matrix metered setting of 1/15sec @ f16, but after
checking the camera review screen and histogram it was clear that the contrast range was too
great for a single exposure. Taking two images at one stop above and
one stop below the meter reading gave much better results for the shadow and
highlight regions respectively. These two images were then blended together
in Photoshop to give the first image below.

blended image

final image

The blending process is a relatively simple procedure,
which can be carried out manually, but is much easier using an automated
action. Blending is achieved by placing the two images on separate layers
(light image below the dark image) and then using a layer mask based on the
lower image to control how the two images are blended. When the lower image
is applied as a mask to the upper image the colour image is converted to a
greyscale mask with the brightest areas being white and the darkest areas
black. Since layer masks control the transparency of their associated image
by making black mask areas transparent and white areas opaque the two images
are blended to produce an image with details in the highlights and the
shadows. In the example above the mask made from the shadows exposure is
white in the sky areas so the sky in the upper image remains opaque and is
seen in the final image. Conversely the darker foreground area of the mask
renders the upper image transparent in these areas so that the shadow detail
present in the lower image shows through. To prevent any artefacts arising
during the blending process at sharp edges between shadows and highlights,
the layer mask is softened using a small amount of gaussian blur. Once the
two images have been blended, either the upper or lower image layers can be
fine tuned using levels or curves to fine tune the final image, as shown
above.

The Exposure Blend action automates the basic stages
described above for merging two exposures. The action guides you through
opening two images in the correct order, converts both images to 8-bit (16
bit files cannot be layered in most versions of Photoshop), and then creates
a blurred layer mask to blend the images. To download the action click the
link below. If you are not familiar with loading and running actions, read
the actions guide.